The present invention relates generally to field effect transistor (FET) reference voltage circuits, and more specifically to process, supply voltage, and temperature (PVT) compensation in FET voltage reference circuits.
Voltage reference circuits that are process, supply voltage, and temperature (PVT) independent have numerous applications. Applications for which PVT independent voltage reference circuits are used include for example forward body bias and analog to digital conversion, as well as any circuits which require accurate supply voltages over a wide range of operating and device conditions.
Conventional voltage reference circuits requiring PVT independence have traditionally used diode or bipolar junction transistor (BJT) bandgap reference circuits. Circuits such as these typically require a supply voltage of at least 1.3 volts. As technology improves, and components become smaller, the supply voltage (Vcc) for processors continues to drop. Some current processor are operating with supply voltages of 1.4 volts. This is close to the limit at which diode or BJT bandgap circuits will become ineffective for use as supply voltages.
As processor supply voltages drop, exploration has begun for the use of different technologies to provide lower supply voltages. Metal oxide semiconductor field effect transistors (MOSFETs) in their subthreshold operation have been used to generate bandgap like reference voltages. However, the use of MOSFETs in such voltage reference circuits do not generally offer good process variation independence for the reference voltage. Process variations can lead to potentially large fluctuations in supply voltage.
Transistors such as BJTs and MOSFETs have linear and non-linear dependencies that occur based on a number of factors. Those factors include temperature, process, and supply voltage. If the process changes, the output voltage of the circuit and the way the circuit operates will change. Reasons for the change in output voltage include changes due to devices in the circuit, and changes due to temperature. The impact of change in device behavior on the reference voltage is primarily linear in nature. Changes due to temperature typically include linear and non-linear changes.
Because of the availability of MOSFET devices to operate at voltages less than typical BJT bandgap voltages, and due to the decreasing supply voltages for integrated circuits and especially processors, there is a need in the art for compensation of MOSFET reference voltage circuits due to changes in device behavior.
In one embodiment, a method for providing a MOSFET based reference voltage includes generating a VPTAT with two MOSFETS of different sizes and a first resistance, and generating a reference voltage with a third MOSFET and a second resistance.
In other embodiments, the generated reference voltage is scaled with a scaling circuit to provide arbitrary reference voltages, or the resistances in the reference voltage circuit are automatically adjusted to compensate for changes in device behavior.
In another embodiment, an apparatus for trimming a reference voltage includes a trimming circuit to generate a first voltage, a reference bias generation circuit to generate a second voltage, and a voltage comparator operatively electrically connected to the trimming circuit and to the reference bias generation circuit to compare the first and the second voltages and to generate a plurality of bits to adjust a variable resistance for the reference voltage.
Other embodiments are described and claimed.